PDF Publication Title:
Text from PDF Page: 024
8 1 Introduction Because of the non-uniform distribution of properties in tubes, heat transfer deteriora- tion or enhancement will be observed in specific conditions, which can be affected by heat flux, mass flow rate, inlet temperature, wall roughness, flow direction and tube diameter. The feature of non-uniformity leads to buoyancy force and flow acceleration bringing unique characteristics to the mechanisms but also difficulties with research. However, with the presence of non-dimensional parameters, buoyancy parameter Bo* and flow acceleration parameter KvT , it becomes helpful and convenient to estimate their effects on heat transfer in theoretical and practical analysis. The development of heat transfer and pressure drop correlations is quite essential for gas cooler design, while the tradi- tional single-phase correlations are not accurate here due to the varying properties (This conclusion has been verified by lots of research). It should be noted that the heat transfer correlation is not the same for heating and cooling, as for the gas cooling process. Kras- noshchekov et al [9] first carried out the heat transfer correlation through experimental study. The correlation considered the difference between wall temperature properties and bulk temperature properties which reflects the property variation in radial direction, thus improving the accuracy. Baskov et al [15], Petrov and Popov [16], Yoon et al [17], etc. also proposed different correlations adapting to different conditions showing relative accu- racy. For pressure drop of the cooling process, the correlations proposed by Petrov and Popov [16], etc. were widely used. ● In evaporator (4–1), CO2 vaporizes during the flow boiling process. Compared to con- ventional HFCs, CO2 shows a higher heat transfer coefficient and a lower pressure drop, indicating better overall performance. During the boiling of CO2, the heat transfer coefficient decreases with increasing quality, and a sharp drop occurs in critical quality at which point dryout phenomenon occurs (i.e. the liquid film breaks down). Boiling heat transfer can be affected by heat flux, mass flux and evaporation temperature. In the region with low qualities, nucleate boiling domi- nates the heat transfer process, showing more sensitivity to heat flux rather than mass flux. And after dryout at high qualities, convective boiling is the major heat transfer process, with the heat transfer coefficient being influenced more by mass flux. Further, the dryout quality is mainly determined by mass flux [18, 19]. As for the pressure drop characteristic, it increases with increasing mass flux while decreasing with increasing evaporation temperature [17]. Flow pattern is another interesting topic for two-phase flow. During the boiling process, slug flow can be observed at low qualities and annular flow with entrainment of droplets can be observed at high qualities [20]. Several researchers have focused on the effect of lubricant on heat transfer, which is essential to be discussed in vapor compression cycles. Rieberer [21] concluded that heat transfer can be influenced by compressor lubricant and the nucleate heat transfer may be weakened. Zhao et al [22] concluded that the decrease rate of heat transfer during boiling can be higher with increasing concentration of lubricant. Like the development of correlations in the gas cooling process, evaporation correlation is of the same importance to evaporator design and operation. Boiling flow correlations have been developed a lot but results have shown that generalized correlations were not accurate for CO2, therefore correlations for CO2 should be developed specifically [5]. ThePDF Image | What is a heat pump
PDF Search Title:
What is a heat pumpOriginal File Name Searched:
L-G-0016174002-0052376109.pdfDIY PDF Search: Google It | Yahoo | Bing
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
Heat Pumps CO2 ORC Heat Pump System Platform More Info
| CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com | RSS | AMP |